A honeycomb panel consists of a honeycomb core brazed to two face sheets. Honeycomb panels of various nickel alloys, such as Rene' 41, Inconel 718, or Inconel 625, are frequently used in panels exposed to elevated temperatures, such as the exterior skin of a space vehicle when it reenters the atmosphere or the skin of an airplane exposed to substantial air friction or hot exhaust gasses. As used herein, a nickel alloy is any alloy which contains more Nickel by weight than any other element, and preferably an alloy which contains at least approximately 50% Nickel by weight.
Inconel 718 is the trademark of INCO Alloys International, Inc. for a nickel alloy consisting of approximately 52.5% Nickel, 19% Chromium, 3% Molybdenum, 5.1% Niobium, 0.9% Titanium, 0.5% Aluminum, and 18.5% Iron, by weight. Rene' 41 is the trademark of General Electric Co. for a nickel alloy consisting of approximately 55% Nickel, 19% Chromium, 10% Molybdenum, 11% Cobalt, 3.1% Titanium, 0.09% Carbon, 0.01% Boron, and 1.5% Aluminum, by weight. Inconel 718 and Rene' 41 are precipitation-hardenable and heat-treatable nickel alloys. Inconel 625 is the trademark of INCO Alloys International, Inc. for a nickel alloy consisting of approximately 61% Nickel, 21.5% Chromium, 9% Molybdenum, 3.6% Niobium, 0.2% Titanium, 0.2% Aluminum, 2.5% Iron, and 0.05% Carbon, by weight. Inconel 625 is a solid-solution and nonheat-treatable nickel alloy.
The metal foil used to manufacture the cores for these nickel-alloy panels typically has a thickness of 0.015 inch to 0.025 inch. Prior brazing materials used on such panels were usually nickel-based alloys containing boron, silicon, or other elements which tended to erode the thin foil of these cores. This erosion substantially weakened the core and reduced both the face sheet tension strength and peel strength of the panel. In addition, these prior brazing materials had a relatively low ductility which also reduced the strength of the panels.
Many of the nickel-based brazing materials, especially those formulated to be less erosive to thin sections, are generally available only in powder form, which requires a long and tedious application process. Specifically, a liquid binder is applied to both face sheets and the powder brazing material is dusted onto the binder. Additional layers of binder and brazing material are added until the desired thickness of brazing material is obtained. The core is then placed on top of one of the face sheets. The other face sheet is flipped over and placed on top of the core, and the core and face sheets are placed in the brazing oven.
There are several disadvantages associated with the use of a powder brazing material. First, it is difficult to create a layer of brazing material having a uniform thickness over the entire surface of the face sheets. As a result, additional amounts of brazing material must be used to ensure that there is sufficient brazing material at all locations on the face sheets. Second, when the face sheet is flipped over and placed on top of the core, a portion of the brazing material may fall off the top sheet. This may also happen at any time after the panel is inserted in the furnace. Again, this necessitates the use of additional amounts of brazing material to obtain sufficient brazing material on the top sheet. Third, the liquid binder outgasses during the brazing process, contaminating the braze joint and the furnace atmosphere. This outgassing also requires longer brazing times to bake out the binder from the brazing material.
In manufacturing the panel, both face sheets are usually brazed to the honeycomb core at the same time, with one face sheet on top of the core and the other face sheet underneath the core in the brazing oven. Due to the force of gravity, and other factors, substantial amounts of brazing material frequently flow down the core away from the top face sheet as the brazing material melts. However, this flow of brazing material away from the top sheet is generally not uniform at all locations on the core. As a result, it is very difficult to uniformly braze the top face sheet to the core. Even if additional amounts of the brazing material are applied to the top sheet, smaller than optimum filets frequently result at various locations in the braze joint between the core and the top sheet. Moreover, the additional amount of brazing material increases the weight of the panel and the erosion of the honeycomb core.
It is recognized in the art that oxidizing a metal surface can reduce the flow of a braze material. However, this technique has not been routinely used for honeycomb core in the past, due to the difficulty in achieving a uniform and repeatable oxide layer on the interior surfaces of the core. If a portion of the interior surfaces of the core is not sufficiently oxidized, the flow of the brazing material may not be sufficiently reduced in that portion of the core, or that portion of the core may not be adequately protected from erosion. On the other hand, if a portion of the interior surface of the core is oxidized to too great an extent, that portion of the core can become brittle or otherwise lose its structural strength, or the braze material may not properly adhere to the core, thereby reducing the overall strength of the panel. A process for depositing a uniform oxide layer must also be repeatable from core to core. This lack of process repeatability has been a significant problem in the past.
Even if the interior surface of the core is uniformly oxidized to the proper extent, the oxidized layer is readily altered in the brazing oven. The atmosphere in the brazing oven can readily affect the oxide layer, through sublimation, reduction, or other processes.